VLAN Trunking and STP

Need clarification on configuring trunk ports between switches.

PCA------------SW1 =====SW2---------PCB  
                 =        =
                    =  =

I have a VLAN “100” configured on SW1 and SW2. Both PC-A(SW1) and PC-B(SW2) are in the same VLAN 100.
I have a trunk configured between SW1 and SW2 and no trunk for VLAN 100 configured on SW3.
How does VLAN trunks behave whenever STP topology port state changes(desg, non desg and root port).
In the above case what if my trunk port becomes a non-designated port(blocking).
What are few of best practices for trunk ports with respect to STP.

Hi @essesss,

You can find some great information here in the introduction-to-spanning-tree lesson. As you know, STP works to shutdown ports that might create a loop. In the original spanning tree specification “STP” , ports were closed regardless of VLAN. later switches became VLAN aware and were able to allow a port to forward on a per-VLAN basis. This is called PVST. MST also supports per-VLAN topology.

Although it might not be immediately obvious, the majority of ports in a normal Ethernet network that participate in spanning tree, will be configured as trunks. To put it another way, spanning tree is designed for trunks!

Kind regards,